Over the years a variety of catalysts have been employed in the production of high-boiling esters. Strong mineral acids, such as sulphuric acid and phosphoric acid, and organic acids such as para-toluene sulphonic acid, have been used extensively. Notwithstanding their extensive use, it has long been recognized that there are problems associated with the use of these catalysts.
In order to achieve commercially viable production rates at temperatures low enough to produce commercially acceptable products, it is necessary to employ such high acid concentrations that these concentrations often cause dehydration of the alcohol reactant. This gives rise to the production of unwanted olefins and ethers, discoloration of the ester product, and an extensive washing step at the end to remove the residual acidity.
Problems caused by the use of strong acid catalysts are known to be alleviated to a great extent by the use of amphoteric catalysts instead of strong acid catalysts. Amphoteric catalysts may be described as compounds of elements capable of functioning as both cations and anions.
The catalyst of the present invention, in a preferred embodiment, comprises a mixture of stannous oxalate and a sodium aluminate in a carrier such as sand. It should be noted that silica sand, ordinarily made up of negatively charged particles, is capable of transporting metallic ions.
In the prior art, it has been known to use stannous oxalate as an active catalytic ingredient to catalyze an esterification reaction. British patent publication GB 990,927 discloses catalysts wherein the active ingredient is a stannous salt of a carboxylic acid.
Also it has been known to use a sodium aluminate as the active ingredient to catalyze esterification reactions. U.S. Pat. No. 3,907,709 of List et al. describes silica-gel catalysts which comprise as the active ingredient a sodium compound, or an aluminum compound, or a tin compound. In their example, List et al. describe silica gel (the carrier) impregnated with a sodium aluminate (the active catalytic material).
However, up until now, it has not been known to use a catalyst which comprises a blend of the two active ingredients, stannous oxalate and a sodium aluminate in an inert carrier.